1. Field of the Invention
The present invention relates to: an optical scanning element having a micro mirror displaced by a physical action force; a driving method for the same; and an optical scanning probe employing this optical scanning element. In particular, the present invention relates to a technique preferably applied to a diagnostic probe of an endoscope device for medical application.
2. Description of Related Art
In recent years, as means for observing the minute situation of the surface and the inside a body tissue and a cell, for example, a confocal optical microscope of optical scanning type is known in which an optical scanning element is mounted. It is even discussed that a similar technique employing an optical scanning element is applied to an endoscope. Such a microscope and an endoscope of optical scanning type have advantages that a resolving power exceeding the resolution limit of a general optical system is achieved and that a three dimensional image can be constructed.
As an optical scanning element to be adopted in such application, various types are proposed. In particular, an optical scanning element in which a micro mirror fabricated by micromachining technology is held on a rotational axis of a twist beam and then reciprocally oscillated by an electrostatic force is hopeful from a practical point of view. A detailed example of this optical scanning element is disclosed in IBM J. Res. Develop Vol. 24 (1980). In this configuration, a mirror substrate supported by two beams provided on the same straight line is reciprocally oscillated by an electrostatic attractive force generated relative to an electrode provided at a position opposing to the mirror substrate, in a state that the two beams serve as a twist rotational axis.
In comparison with an optical scanning element employing a rotating polygon mirror in the related art, an optical scanning element formed by micromachining technology has a simple structure and hence can be integrally formed in a semiconductor process. Thus, size reduction is easy and the manufacturing cost is low. Further, because of a single reflecting surface, variation in the precision depending on the individual reflecting surfaces is avoided which could occur in the case of a polygon mirror. Furthermore, the reciprocal scanning permits speed improvement.
As optical scanning elements described above, for example, structures disclosed in JP-A-2000-310743, JP-A-11-52278, and JP-A-2005-141229 are known.
FIGS. 30A and 30B show configurations of optical scanning elements disclosed in JP-A-2000-310743. The optical scanning elements 1A and 1B have configurations for application to an endoscope. The drive system adopted for each optical scanning element is of general electrostatic drive. That is, as shown in FIG. 30A, a voltage is applied between movable electrodes 3a and 3b provided in the left and right parts of a movable section 2 and a fixed electrode not illustrated but installed in an opposing manner. Then, a generated electrostatic force causes rotational displacement in the movable section 2 about a beam 4 serving as the center axis. The shaded region in the figure indicates a mirror part. FIG. 31A shows a configuration of a one-axis mirror. FIG. 31B shows a configuration of a two-axis mirror that performs rotational displacement about the beams 4 and 5 serving as the center axes when a voltage is applied on movable electrodes 3a and 3b and movable electrodes 3c and 3d. 
FIG. 31 shows a configuration of an optical scanning element disclosed in JP-A-11-52278. The configuration of the optical scanning element 1C shown in FIG. 31 is, in general, referred to as a comb-tooth electrode structure. That is, the distance between electrodes is small, while the facing area of the electrodes is enhanced. This permits voltage reduction in the driving voltage. Further, since fixed electrodes 6a and 6b are not located in the direction of displacement of a movable section 2 (a direction perpendicular to the figure), a pull-in phenomenon does not occur that the movable section 2 sticks to the fixing electrodes 6a and 6b. This permits a larger scanning angle.
FIG. 32 shows a configuration of an optical scanning element disclosed in JP-A-2005-141229. The optical scanning element 1D shown in FIG. 32 has a comb-tooth electrode structure, similarly to that disclosed in JP-A-2005-141229. Thus, the distance between electrodes is small, while the opposing area of the electrodes is enhanced. This permits voltage reduction in the driving voltage. Further, since fixed electrodes 7a, 7b, 8a, and 8b are not located in the direction of displacement of a movable section 2, a pull-in phenomenon does not occur. This permits a larger scanning angle.
Meanwhile, when an optical scanning element is to be mounted on an endoscope, since the diameter of a probe to be inserted into the forceps port of the endoscope is extremely small, the optical scanning element need be constructed in a remarkably small size. Further, when a confocal optical system is adopted, scanning drive for the optical scanning element need be in two individual axial directions. Nevertheless, in the element shape of an optical scanning element according to the prior art, the ratio of the area occupied by electrodes and support parts to the area of a reflecting surface is comparatively large, and hence the aperture ratio of the reflecting surface is small. This has avoided that the merit of minute fabrication adopting a MEMS (micro electro mechanical system) technique is fully utilized.
Further, in the technique described in JP-A-2000-310743, a support drive region for supporting a mirror need be ensured in the surrounding of the mirror (a shaded region in the figure) serving as a reflecting surface. Thus, the area around the reflecting surface is large. In particular, in the case of a two-axis mirror, support drive regions for the two axes need be ensured. Thus, the ratio (aperture ratio) of the area of the reflecting surface to the area of the entire element is reduced unavoidably. When the optical scanning element is to be installed in the tip part of a thin endoscope probe, the size of the entire element is limited. Thus, the reflecting surface is small, and hence the aperture ratio cannot be high. Further, the size of the movable section 2 is also limited. Thus, available flexibility is low in the element design.
Further, in the technique described in JP-A-11-52278, the electrodes having a comb-tooth structure are located in the horizontal direction of the movable section. Thus, a region used for arranging the comb-tooth electrodes need be ensured in addition to that for the support part. This reduces the aperture ratio further. When the electrode area is reduced, the applied voltage need be increased. This causes difficulty in realizing low voltage drive. Further, in particular, in application to a device such as an endoscope to be inserted into a body, the use of a high voltage is undesired. This causes a problem of insufficient function.
Further, in the technique described in JP-A-2005-141229, similarly to the case of JP-A-11-52278, the electrodes 7a, 7b, 8a, and 8b having a comb-tooth structure are arranged around the movable section 2. Thus, a region used for arranging the comb-tooth electrodes 7a, 7b, 8a, and 8b need be ensured in addition to that for the support part. As such, when a two-axis mirror is to be realized by adopting comb-tooth electrodes, the area necessary for arranging comb-tooth electrodes increases. This reduces the aperture ratio yet further.